1. Field of the Invention
The invention is directed to a method and apparatus for measuring web strain in a confined area.
2. Description of the Related Art
It is known that visual access to the web and carton forming area is hindered by packaging machinery, making web strain measurement very difficult. Furthermore, the geometry of a web changes considerably during converting operations, thereby making traditional optical strain measurement techniques unworkable.
It is known to use photographs to measure web strain. Exemplary of such prior art are U.S. Pat. No. 5,757,473 (""473) to H. Kanduth et al., entitled xe2x80x9cOptical Strain Sensor for the Measurement of Microdeformations of Surfaces,xe2x80x9d U.S. Pat. No. 5,812,251 (""251) to A. Manesh, entitled xe2x80x9cElectro-Optic Strain Gages and Transducer,xe2x80x9d and U.S. Pat. No. 5,920,383 (""383) to F. Chen et al., entitled xe2x80x9cMicroscopic Digital Imagining Strain Gauge.xe2x80x9dThe ""473, ""251, and ""383 references teach photographing the web in strained and unstrained conditions; however, they are not readily adaptable to confined areas. Access to the web must be unhindered for the microscope lenses, prisms, surface attachments, etc. Thus making the conventional techniques unacceptable in confined areas.
Web strain measurements in more confined areas can be conventionally made using digital cameras and conventional fiber optic scopes to photograph a web area with marked reference points. FIG. 1 illustrates a conventional narrow fiber optical scope 2 that can be fitted with a camera (not shown). Scope 2 includes a conventional fiber optic 4. The end of the fiber optic 7 is placed at a known focal length from an object 50. Exemplary of such prior art is U.S. Pat. No. 5,726,907 (""907) to D. L. Davidson et al., entitled xe2x80x9cBi-Axial Non-Contacting Strain Measurement Using Machine Vision.xe2x80x9d The ""907 reference teaches a method whereby a digital camera photographs a sample area having marked reference points marked. Any subsequent sample strain is determined by comparing the relative movement of the reference points from one another. Using a digital computer, strain measurement may be automated. The above method works well for one-directional strain. However, for bi-axial strains the geometry, e.g. focal length, between the camera and web must be maintained throughout the measurement process. If the focal length and/or incident angle of the focal axis changes in an uncertain manner, the web strain cannot accurately be measured.
It is apparent from the above that there exists a need in the art for a method and apparatus for measuring web strain in a confined area.
The invention fulfills these needs by providing a method and apparatus for measuring web strain in a confined area. The apparatus comprises a fiber optic scope means and a fiber optic tube means operatively connected to the fiber optic scope means. The fiber optic scope means is secured inside the fiber optic tube means. An adjustable stand-off means is placed on the web. In an exemplary embodiment the stand-off means is operatively connected to the fiber optic tube means to provide a fixed but adjustable focal length between an end of the fiber optic scope means and the surface of the web.
In an exemplary method, the web surface is marked with a series of reference points. The reference points are visually located using the fiber optic scope means. After the web undergoes a movement, such as a converting step or fold, the web is again visually examined using the fiber optic scope means. The relative displacement of the reference points from the web movement is visually determined.